Distributed sensing may be useful in the structural health monitoring of civil infrastructure, airframes, and process control systems. Distributed sensing typically utilizes wireless sensors and transmission networks. Providing power to the wireless sensors requires either long-term power sources, or devices that can harvest power from ambient or driven vibrations.
The three principal mechanisms of converting mechanical energy to electrical energy include electrostatic, electromagnetic, and piezoelectric conversions. For example, piezoelectric devices principally fabricated as cantilevers of ceramic (e.g., lead zirconate-titanate (PZT)) have been developed. In devices utilizing these mechanisms, however, extracting the maximum power from ambient vibration relies upon maintaining system vibration at a resonance frequency.
Two methods of controlling the resonant frequency of vibrating piezoelectric structures include:                1. Control the stiffness of a piezoelectric component by adaptive capacitive loading or other electrical control schemes; or        2. Clamp a beam at both ends and apply a destabilizing compressive load, which decreases the effective transverse stiffness of the beam as it approaches the buckling condition.However, the foregoing techniques may not provide sufficient tunability for certain applications (e.g., the distributed sensing applications). Accordingly, energy converters with improved resonant frequency modulation are needed.        